Method and apparatus for entropy coding video and method and apparatus for entropy decoding video

ABSTRACT

Provided are entropy decoding and encoding methods of a video. The entropy decoding method includes obtaining a transformation unit significant coefficient flag indicating whether a non-zero transformation coefficient exists in the transformation unit, from a bitstream, determining a context model for arithmetically decoding the transformation unit significant coefficient flag, based on the transformation depth of the transformation unit and arithmetically decoding the transformation unit significant coefficient flag based on the determined context model.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 14/362,771, which isa National Stage application under 35 U.S.C. §371 of PCT/KR2013/005870,filed on Jul. 2, 2013, which claims priority from U.S. ProvisionalApplication No. 61/667,117, filed on Jul. 2, 2012, all the disclosuresof which are incorporated herein in their entireties by reference.

BACKGROUND

1. Field

Exemplary embodiments relate to video encoding and decoding. Inparticular, exemplary embodiments relate to a method and apparatus forentropy encoding and decoding information related to a transformationunit.

2. Related Art

According to image compression methods such as MPEG-1, MPEG-2, or MPEG-4H.264/MPEG-4 advanced video coding (AVC), an image is split into blockshaving a predetermined size. Then, residual data of the blocks areobtained by inter prediction or intra prediction. Residual data iscompressed by transformation, quantization, scanning, run length coding,and entropy coding. In entropy coding, a syntax element such as atransformation coefficient or a prediction mode is entropy encoded tooutput a bitstream. A decoder parses and extracts syntax elements from abitstream, and reconstructs an image based on the extracted syntaxelements.

SUMMARY

Exemplary embodiments may include an entropy encoding method andapparatus, and an entropy decoding method and apparatus for selecting acontext model used to entropy encode and decode a syntax element relatedto a transformation unit that is a data unit used to transform a codingunit, based on a transformation depth indicating a hierarchicalsplitting relationship between the coding unit and the transformationunit.

A context model for arithmetically decoding a transformation unitsignificant coefficient flag is determined based on a transformationdepth indicating the number of times the coding unit is split todetermine the transformation unit included in the coding unit, and thetransformation unit significant coefficient flag is arithmeticallydecoded based on the determined context model.

According to exemplary embodiments, by selecting a context model basedon a transformation depth, a condition for selecting the context modelmay be simplified and operation for entropy encoding and decoding mayalso be simplified.

According to an aspect of an exemplary embodiment, an entropy decodingmethod of a video is provided, the method includes determining atransformation unit which is included in a coding unit and is used toinversely transform the coding unit; obtaining a transformation unitsignificant coefficient flag from a bitstream which indicates whether anon-zero transformation coefficient exists in the transformation unit;determining a context model for arithmetically decoding thetransformation unit significant coefficient flag based on atransformation depth of the transformation unit in response to a numberof times the coding unit is split when determining the transformationunit being the transformation depth of the transformation unit; andarithmetically decoding the transformation unit significant coefficientflag based on the determined context model.

According to an aspect of an exemplary embodiment, an entropy decodingapparatus of a video is provided, the apparatus includes a parserconfigured to obtain a transformation unit significant coefficient flagfrom a bitstream which indicates whether a non-zero transformationcoefficient exists in a transformation unit which is included in acoding unit and used to inversely transform the coding unit; a contextmodeler configured to determine a context model for arithmeticallydecoding the transformation unit significant coefficient flag based on atransformation depth of the transformation unit in response to a numberof times the coding unit is split when determining the transformationunit being the transformation depth of the transformation unit; and anarithmetic decoder configured to arithmetically decode thetransformation unit significant coefficient flag based on the determinedcontext model.

According to an aspect of an exemplary embodiment, an entropy encodingmethod of a video is provided, the method includes: obtaining data of acoding unit transformed based on a transformation unit; determining acontext model for arithmetically encoding a transformation unitsignificant coefficient flag which indicates whether a non-zerotransformation coefficient exists in the transformation unit based on atransformation depth of the transformation unit in response to a numberof times the coding is split when determining the transformation unitbeing the transformation depth of the transformation unit; andarithmetically encoding the transformation unit significant coefficientflag based on the determined context model.

According to an aspect of an exemplary embodiment, an entropy encodingapparatus of a video is provided, the apparatus includes: a contextmodeler configured to obtain data of a coding unit which is transformedbased on a transformation unit and, determine a context model forarithmetically encoding a transformation unit significant coefficientflag indicating whether a non-zero transformation coefficient exists inthe transformation unit based on a transformation depth of thetransformation unit in response to a number of times the coding unit issplit when determining the transformation unit being the transformationdepth of the transformation unit; and an arithmetic encoder configuredto arithmetically encode the transformation unit significant coefficientflag based on the determined context model.

According to an aspect of an exemplary embodiment, a video decodingapparatus is provided, the video decoding apparatus includes a parserconfigured to receive a bitstream of an encoded video and parse at leastone syntax element; an entropy decoder configured to arithmeticallydecode the at least one parsed syntax element by performing entropydecoding of the at least one parsed syntax element and extractinformation about at least one of a coded depth and an encoding modeaccording to each largest coding unit from the at least one parsedsyntax element; and a hierarchical decoded configured to reconstruct apicture by decoding image data in each largest coding unit based on theat least one of the coded depth and the encoded mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a video encoding apparatus according to anexemplary embodiment.

FIG. 2 is a block diagram of a video decoding apparatus according to anexemplary embodiment.

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

FIG. 4 is a block diagram of a video encoder based on coding unitshaving a hierarchical structure, according to an exemplary embodiment.

FIG. 5 is a block diagram of a video decoder based on coding unitshaving a hierarchical structure, according to an exemplary embodiment.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

FIG. 7 is a diagram for describing a relationship between a coding unitand transformation units, according to an exemplary embodiment.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units, prediction units, and frequency transformation units,according to an exemplary embodiment.

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit, and a transformation unit, according toencoding mode information of Table 1.

FIG. 14 is a block diagram of an entropy encoding apparatus according toan exemplary embodiment.

FIG. 15 is a flowchart of an operation of entropy encoding and decodinga syntax element related to a transformation unit, according to anexemplary embodiment.

FIG. 16 is a diagram illustrating a coding unit and transformation unitsincluded in the coding unit, according to an exemplary embodiment.

FIG. 17 is a diagram illustrating a context increasement parameter usedto determine a context model of a transformation unit significantcoefficient flag of each of the transformation units of FIG. 16, basedon a transformation depth.

FIG. 18 is a diagram illustrating a coding unit and a transformationunit included in the coding unit, according to another exemplaryembodiment.

FIG. 19 is a diagram illustrating split transformation flags used todetermine the structure of transformation units included in the codingunit of FIG. 16, according to an exemplary embodiment.

FIG. 20 illustrates a transformation unit that is entropy encodedaccording to an exemplary embodiment.

FIG. 21 illustrates a significance map corresponding to thetransformation unit of FIG. 20.

FIG. 22 illustrates coeff_abs_level_greater1_flag corresponding to the4×4 transformation unit of FIG. 20.

FIG. 23 illustrates coeff_abs_level_greater2_flag corresponding to the4×4 transformation unit of FIG. 20.

FIG. 24 illustrates coeff_abs_level_remaining corresponding to the 4×4transformation unit of FIG. 20.

FIG. 25 is a flowchart of an entropy encoding method of a video,according to an exemplary embodiment.

FIG. 26 is a block diagram of an entropy decoding apparatus according toan exemplary embodiment.

FIG. 27 is a flowchart of an entropy decoding method of a video,according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, a method and apparatus for updating a parameter used inentropy encoding and decoding size information of a transformation unitaccording to an exemplary embodiment of will be described with referenceto FIGS. 1 through 13. In addition, a method of entropy encoding anddecoding a syntax element obtained by using the method of entropyencoding and decoding of a video described with reference to FIGS. 1through 13 will be described in detail with reference to FIGS. 14through 27. Expressions such as “at least one of,” when preceding a listof elements, modify the entire list of elements and do not modify theindividual elements of the list.

FIG. 1 is a block diagram of a video encoding apparatus 100 according toan exemplary embodiment.

The video encoding apparatus 100 includes a hierarchical encoder 110 andan entropy encoder 120.

The hierarchical encoder 110 may split a current picture to be encoded,in units of predetermined data units to perform encoding on each of thedata units. In detail, the hierarchical encoder 110 may split a currentpicture based on a largest coding unit, which is a coding unit of amaximum size. The largest coding unit according to an exemplaryembodiment may be a data unit having a size of 32×32, 64×64, 128×128,256×256, etc., wherein a shape of the data unit is a square which haswidth and length in squares of 2 and is greater than 8.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes the number of times thecoding unit is spatially split from the largest coding unit, and as thedepth deepens, deeper encoding units according to depths may be splitfrom the largest coding unit to a smallest coding unit. A depth of thelargest coding unit is an uppermost depth and a depth of the smallestcoding unit is a lowermost depth. Since a size of a coding unitcorresponding to each depth decreases as the depth of the largest codingunit deepens, a coding unit corresponding to an upper depth may includea plurality of coding units corresponding to lower depths.

As described above, image data of the current picture is split into thelargest coding units according to a maximum size of the coding unit, andeach of the largest coding units may include deeper coding units thatare split according to depths. Since the largest coding unit accordingto an exemplary embodiment is split according to depths, the image dataof a spatial domain included in the largest coding unit may behierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the largest coding unitare hierarchically split, may be predetermined.

The hierarchical encoder 110 encodes at least one split region obtainedby splitting a region of the largest coding unit according to depths,and determines a depth to output finally encoded image data according tothe at least one split region. In other words, the hierarchical encoder110 determines a coded depth by encoding the image data in the deepercoding units according to depths, according to the largest coding unitof the current picture, and selecting a depth having the least encodingerror. The determined coded depth and the encoded image data accordingto maximum encoding units are output to the entropy encoder 120.

The image data in the largest coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or less thanthe maximum depth, and results of encoding the image data are comparedbased on each of the deeper coding units. A depth having the leastencoding error may be selected after comparing encoding errors of thedeeper coding units. At least one coded depth may be selected for eachlargest coding unit.

The size of the largest coding unit is split as a coding unit ishierarchically split according to depths and as the number of codingunits increases. Also, even if coding units correspond to a same depthin one largest coding unit, it is determined whether to split each ofthe coding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of each coding unit,separately. Accordingly, even when image data is included in one largestcoding unit, the image data is split into regions according to thedepths, and the encoding errors may differ according to regions in theone largest coding unit, and thus the coded depths may differ accordingto regions in the image data. Thus, one or more coded depths may bedetermined in one largest coding unit, and the image data of the largestcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the hierarchical encoder 110 may determine coding unitshaving a tree structure included in the largest coding unit. The ‘codingunits having a tree structure’ according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among all deeper coding units included in the largest codingunit. A coding unit having a coded depth may be hierarchicallydetermined according to depths in the same region of the largest codingunit, and may be independently determined in different regions.Similarly, a coded depth in a current region may be independentlydetermined from a coded depth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of times a largest coding unit is split into smallestcoding units. A first maximum depth according to an exemplary embodimentmay denote the total number of times the largest coding unit is splitinto the smallest coding units. A second maximum depth according to anexemplary embodiment may denote the total number of depth levels fromthe largest coding unit to the smallest coding unit. For example, when adepth of the largest coding unit is 0, a depth of a coding unit, inwhich the largest coding unit is split once, may be set to 1, and adepth of a coding unit, in which the largest coding unit is split twice,may be set to 2. If the smallest coding unit is a coding unit in whichthe largest coding unit is split four times, five depth levels of depths0, 1, 2, 3, and 4 exist. Thus, the first maximum depth may be set to 4,and the second maximum depth may be set to 5.

Prediction encoding and transformation may be performed according to thelargest coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the largestcoding unit.

Since the number of deeper coding units increases whenever the largestcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For descriptiveconvenience, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a largest codingunit.

The video encoding apparatus 100 may variously select a size or shape ofa data unit for encoding the image data. In order to encode the imagedata, operations, such as prediction encoding, transformation, andentropy encoding, are performed, and at this time, the same data unitmay be used for all operations or different data units may be used foreach operation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data, but also a data unit differentfrom the coding unit so as to perform the prediction encoding on theimage data in the coding unit.

In order to perform prediction encoding in the largest coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split into coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will be referred to as a ‘prediction unit’.A partition obtained by splitting the prediction unit may include aprediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, a size of apartition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition typeinclude symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, an inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having the leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data, but also based on a data unit that is differentfrom the coding unit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a size equalto or less than the size of the coding unit. For example, the data unitfor the transformation may include a data unit for an intra mode and adata unit for an inter mode.

A data unit used as a base of the transformation is referred to as a‘transformation unit’. Similar to the coding unit, the transformationunit in the coding unit may be recursively split into smaller sizedregions, so that the transformation unit may be determined independentlyin units of regions. Thus, residual data in the coding unit may bedivided according to the transformation unit having the tree structureaccording to transformation depths.

A transformation depth indicating the number of times the height andwidth of the coding unit are split to reach the transformation unit mayalso be set in the transformation unit. For example, in a current codingunit of 2N×2N, a transformation depth may be 0 when the size of atransformation unit is 2N×2N, may be 1 when the size of a transformationunit is N×N, and may be 2 when the size of a transformation unit isN/2×N/2. That is, the transformation unit having the tree structure mayalso be set according to transformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the hierarchical encoder 110 not only determines a codeddepth having the least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transformation unit for transformation.

Coding units according to a tree structure in a largest coding unit anda method of determining a partition, according to exemplary embodiments,will be described in detail below with reference to FIGS. 3 through 12.

The hierarchical encoder 110 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The entropy encoder 120 outputs the image data of the largest codingunit, which is encoded based on the at least one coded depth determinedby the hierarchical encoder 110, and information about the encoding modeaccording to the coded depth, in bitstreams. The encoded image data maybe a coding result of residual data of an image. The information aboutthe encoding mode according to the coded depth may include informationabout the coded depth, information about the partition type in theprediction unit, prediction mode information, and size information ofthe transformation unit. In particular, as will be described below, theentropy encoder 120 may entropy encode a transformation unit significantcoefficient flag (coded_block_flag) cbf indicating whether a non-0transformation coefficient is included in a transformation unit, using acontext model determined based on a transformation depth of thetransformation unit. An operation of entropy encoding syntax elementsrelated to a transformation unit in the entropy encoding unit 120 willbe described below.

The information about the coded depth may be defined using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output. Thus, thesplit information may be defined not to split the current coding unit toa lower depth. Alternatively, if the current depth of the current codingunit is not the coded depth, the encoding is performed on the codingunit of the lower depth. Thus, the split information may be defined tosplit the current coding unit to obtain the coding units of the lowerdepth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth. Thus, the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onelargest coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one largest coding unit.Also, a coded depth of the image data of the largest coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths. Thus, information about the coded depth andthe encoding mode may be set for the image data.

Accordingly, the entropy encoder 120 may assign encoding informationabout a corresponding coded depth and an encoding mode to at least oneof the coding unit, the prediction unit, and a minimum unit included inthe largest coding unit.

The minimum unit according to an exemplary embodiment is a square-shapeddata unit obtained by splitting the smallest coding unit constitutingthe lowermost depth by 4. Alternatively, the minimum unit may be amaximum square-shaped data unit that may be included in all of thecoding units, prediction units, partition units, and transformationunits included in the largest coding unit.

For example, the encoding information output through the entropy encoder120 may be classified into encoding information according to codingunits and encoding information according to prediction units. Theencoding information according to the coding units may include theinformation about the prediction mode and about the size of thepartitions. The encoding information according to the prediction unitsmay include information about an estimated direction of an inter mode,about a reference image index of the inter mode, about a motion vector,about a chroma component of an intra mode, and about an interpolationmethod of the intra mode. Also, information about a maximum size of thecoding unit defined according to pictures, slices, or group of pictures(GOPs), and information about a maximum depth may be inserted into aheader of a bitstream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or width of a coding unit ofan upper depth, which is one layer above, by two. In other words, whenthe size of the coding unit of the current depth is 2N×2N, the size ofthe coding unit of the lower depth is N×N. Also, the coding unit of thecurrent depth having the size of 2N×2N may include a maximum number offour coding units of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each largest coding unit, based on thesize of the largest coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each largest coding unit using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having a high resolution or a large data amount isencoded in a related art macroblock, a number of macroblocks per pictureexcessively increases. Accordingly, a number of pieces of compressedinformation generated for each macroblock increases. Thus, it isdifficult to transmit the compressed information, and data compressionefficiency decreases. However, by using the video encoding apparatus100, image compression efficiency may be increased since a coding unitis adjusted while considering characteristics of an image whileincreasing a maximum size of a coding unit while considering a size ofthe image.

FIG. 2 is a block diagram of a video decoding apparatus 200 according toan exemplary embodiment.

The video decoding apparatus 200 includes a parser 210, an entropydecoder 220, and a hierarchical decoder 230. Definitions of variousterms, such as a coding unit, a depth, a prediction unit, atransformation unit, and information about various encoding modes, forvarious operations of the video decoding apparatus 200 are identical tothose described with reference to FIG. 1 and the video encodingapparatus 100.

The parser 210 receives a bitstream of an encoded video to parse asyntax element. The entropy decoder 220 arithmetically decodes syntaxelements indicating encoded image data based on coding units having astructure by performing entropy decoding of parsed syntax elements, andoutputs the arithmetically decoded syntax elements to the hierarchicaldecoder 230. In other words, the entropy decoder 220 performs entropydecoding of syntax elements that are received in the form of bit stringsof 0 and 1, thereby reconstructing the syntax elements.

Also, the entropy decoder 220 extracts information about a coded depth,an encoding mode, color component information, prediction modeinformation, etc., for the coding units having a tree structureaccording to each largest coding unit, from the parsed bitstream. Theextracted information about the coded depth and the encoding mode isoutput to the hierarchical decoder 230. The image data in a bitstream issplit into the largest coding unit so that the hierarchical decoder 230may decode the image data for each largest coding unit.

The information about the coded depth and the encoding mode according tothe largest coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about a partition type of acorresponding coding unit corresponding to the coded depth, about aprediction mode, and a size of a transformation unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach largest coding unit extracted by the entropy decoder 220 isinformation about a coded depth and an encoding mode determined togenerate a minimum encoding error when an encoder, such as the videoencoding apparatus 100, repeatedly performs encoding for each deepercoding unit according to depths according to each largest coding unit.Accordingly, the video decoding apparatus 200 may reconstruct an imageby decoding the image data according to a coded depth and an encodingmode that generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the entropy decoder220 may extract the information about the coded depth and the encodingmode according to the predetermined data units. When information about acoded depth and encoding mode of a corresponding largest coding unit isassigned to each of predetermined data units, the predetermined dataunits to which the same information about the coded depth and theencoding mode is assigned may be inferred to be the data units includedin the same largest coding unit.

Also, as will be described below, the entropy decoder 220 may entropydecode a transformation unit significant coefficient flag cbf using acontext model determined based on a transformation depth of atransformation unit. An operation of entropy decoding syntax elementsrelated to a transformation unit in the entropy decoder 220 will bedescribed below.

The hierarchical decoder 230 reconstructs the current picture bydecoding the image data in each largest coding unit based on theinformation about the coded depth and the encoding mode according to thelargest coding units. In other words, the hierarchical decoder 230 maydecode the encoded image data based on the extracted information aboutthe partition type, the prediction mode, and the transformation unit foreach coding unit from among the coding units having the tree structureincluded in each largest coding unit. A decoding operation may includeprediction including intra prediction and motion compensation, andinverse transformation.

The hierarchical decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the hierarchical decoder 230 may perform inverse transformationaccording to each transformation unit in the coding unit, based on theinformation about the size of the transformation unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to largest coding units.

The hierarchical decoder 230 may determine at least one coded depth of acurrent largest coding unit using split information according to depths.If the split information indicates that image data is no longer split inthe current depth, the current depth is a coded depth. Accordingly, thehierarchical decoder 230 may decode the coding unit of the current depthwith respect to the image data of the current largest coding unit usingthe information about the partition type of the prediction unit, theprediction mode, and the size of the transformation unit.

In other words, data units containing the encoding information includingthe same split information may be collected by observing the setencoding information assigned for the predetermined data unit from amongthe coding unit, the prediction unit, and the minimum unit, and thecollected data units may be considered to be one data unit to be decodedby the hierarchical decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each largest coding unit, and may use theinformation to decode the current picture. In other words, encoded imagedata of the coding units having the tree structure determined to be theoptimum coding units in each largest coding unit may be decoded.

Accordingly, even if image data has a high resolution and a large amountof data, the image data may be efficiently decoded and reconstructedusing a size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit, according to an exemplaryembodiment, will now be described with reference to FIGS. 3 through 13.

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

As shown in FIG. 3, a size of a coding unit may be expressed inwidth×height, and may be 64×64, 32×32, 16×16, and 8×8. A coding unit of64×64 may be split into partitions of 64×64, 64×32, 32×64, or 32×32. Acoding unit of 32×32 may be split into partitions of 32×32, 32×16,16×32, or 16×16. A coding unit of 16×16 may be split into partitions of16×16, 16×8, 8×16, or 8×8. A coding unit of 8×8 may be split intopartitions of 8×8, 8×4, 4×8, or 4×4.

Regarding video data 310, a resolution of 1920×1080, a maximum size of acoding unit of 64, and a maximum depth of 2 are set. Regarding videodata 320, a resolution of 1920×1080, a maximum size of a coding unit of64, and a maximum depth of 3 are set. Regarding video data 330, aresolution of 352×288, a maximum size of a coding unit of 16, and amaximum depth of 1 are set. The maximum depth shown in FIG. 3 denotes atotal number of splits from a largest coding unit to a smallest codingunit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe vide data 310 may include a largest coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the largest coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a largest coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the largestcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a largest coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the largestcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 4 is a block diagram of a video encoder 400 based on coding unitshaving a hierarchical structure, according to an exemplary embodiment.

An intra predictor 410 performs intra prediction on coding units in anintra mode, with respect to a current frame 405, and a motion estimator420 and a motion compensator 425 respectively perform inter estimationand motion compensation on coding units in an inter mode using thecurrent frame 405 and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is reconstructed as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and thereconstructed data in the spatial domain is output as the referenceframe 495 after being post-processed through a deblocking filter 480 anda loop filter 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

The entropy encoding unit 450 arithmetically encodes syntax elementsrelated to a transformation unit, such as a transformation unitsignificant coefficient flag (cbf) indicating whether a non-0transformation coefficient is included in a transformation unit, asignificance map indicating a location of a non-0 transformationcoefficient, a first critical value flag (coeff_abs_level_greater1_flag)indicating whether a transformation coefficient has a value greater than1, a second critical value flag (coeff_abs_level_greather2_flag)indicating whether a transformation coefficient has a value greater than2, and a size information of a transformation coefficient(coeff_abs_level_remaining) corresponding to a difference between a baselevel (baseLevel) that is determined based on the first critical valueflag and the second critical value flag and a real transformationcoefficient (abscoeff).

In order for the video encoder 400 to be applied in the video encodingapparatus 100, all elements of the video encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocking filter 480,and the loop filter 490, have to perform operations based on each codingunit from among coding units having a tree structure while consideringthe maximum depth of each largest coding unit.

In particular, the intra predictor 410, the motion estimator 420, andthe motion compensator 425 determine partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentlargest coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

FIG. 5 is a block diagram of a video decoder 500 based on coding units,according to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding, from a bitstream 505. The encodedimage data passes through the decoder 520 and the inverse quantizer 530to be output as inversely quantized data. The entropy decoder 520obtains elements related to a transformation unit from a bitstream, thatis, a transformation unit significant coefficient flag (cbf) indicatingwhether a non-0 transformation coefficient is included in atransformation unit, a significance map indicating a location of a non-0transformation coefficient, a first critical value flag(coeff_abs_level_greater1_flag) indicating whether a transformationcoefficient has a value greater than 1, a second critical value flag(coeff_abs_level_greather2_flag) indicating whether a transformationcoefficient has a value greater than 2, and a size information of atransformation coefficient (coeff_abs_level_remaining) corresponding toa difference between a base level (baseLevel) that is determined basedon the first critical value flag and the second critical value flag anda real transformation coefficient (abscoeff), and arithmetically decodesthe obtained syntax elements so as to reconstruct the syntax elements.

An inverse transformer 540 reconstructs the inversely quantized data toimage data in a spatial domain. An intra predictor 550 performs intraprediction on coding units in an intra mode with respect to the imagedata in the spatial domain, and a motion compensator 560 performs motioncompensation on coding units in an inter mode using a reference frame585.

The image data in the spatial domain, which has passed through the intrapredictor 550 and the motion compensator 560, may be output as areconstructed frame 595 after being post-processed through a deblockingfilter 570 and a loop filter 580. Also, the image data, which ispost-processed through the deblocking filter 570 and the loop filter580, may be output as the reference frame 585.

In order for the video decoder 500 to be applied in the video decodingapparatus 200, all elements of the video decoder 500, i.e., the parser510, the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking filter 570, and the loop filter 580, perform operationsbased on coding units having a tree structure for each largest codingunit.

The intra predictor 550 and the motion compensator 560 determine apartition and a prediction mode for each coding unit having a treestructure, and the inverse transformer 540 has to determine a size of atransformation unit for each coding unit.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units so as to consider characteristics of animage. A maximum height, a maximum width, and a maximum depth of codingunits may be adaptively determined according to the characteristics ofthe image, or may be differently set by a user. Sizes of deeper codingunits according to depths may be determined according to thepredetermined maximum size of the coding unit.

In a hierarchical structure 600 of coding units according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are each 64, and the maximum depth is 4. Since a depthdeepens along a vertical axis of the hierarchical structure 600, aheight and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a largest coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a smallest coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsincluded in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similar, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similar, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similar, a prediction unit of the coding unit 640 having the size of 8×8and the depth of 3 may be split into partitions included in the codingunit 640, i.e. a partition having a size of 8×8 included in the codingunit 640, partitions 642 having a size of 8×4, partitions 644 having asize of 4×8, and partitions 646 having a size of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 4 is thesmallest coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partitionhaving a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the largest coding unit 610, the hierarchical encoder 110of the video encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the largest coding unit 610.

The number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths and performing encoding for each depth as the depth deepens alongthe vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the largest coding unit610 may be selected as the coded depth and a partition type of thelargest coding unit 610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an exemplary embodiment.

The video encoding apparatus 100 or the video decoding apparatus 200encodes or decodes an image of each largest coding unit according tocoding units having sizes equal to or less than the size of the largestcoding unit. Sizes of transformation units for transformation duringencoding may be selected based on data units that are not larger than acorresponding coding unit.

For example, in the video encoding apparatus 100 or the video decodingapparatus 200, if a size of the coding unit 710 is 64×64, transformationmay be performed by using the transformation units 720 having a size of32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, and then a transformation unit having the least coding errormay be selected.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

An output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_(—)0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 802 having a size of 2N×2N, the partition 804 having a size of2N×N, the partition 806 having a size of N×2N, and the partition 808having a size of N×N.

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transformation unit to be based on whentransformation is performed on a current coding unit. For example, thetransformation unit may be a first intra transformation unit 822, asecond intra transformation unit 824, a first inter transformation unit826, or a second inter transformation unit 828.

The image data and encoding data extracting unit 210 of the videodecoding apparatus 200 may extract and use the information 800information about coding units, the information 810 about a predictionmode, and the information 820 about a size of a transformation unit, fordecoding, according to each deeper coding unit.

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding of a coding unit 900having a depth of 0 and a size of 2N_(—)0×2N_(—)0 may include partitionsof a partition type 912 having a size of 2N_(—)0×2N_(—)0, a partitiontype 914 having a size of 2N_(—)0×N_(—)0, a partition type 916 having asize of N_(—)0×2N_(—)0, and a partition type 918 having a size ofN_(—)0×N_(—)0. FIG. 9 only illustrates the partition types 912 through918 which are obtained by symmetrically splitting the prediction unit910, but a partition type is not limited thereto. Therefore, thepartitions of the prediction unit 910 may include asymmetricalpartitions, partitions having a predetermined shape, and partitionshaving a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_(—)0×2N_(—)0, two partitions having a size of 2N_(—)0×N_(—)0,two partitions having a size of N_(—)0×2N_(—)0, and four partitionshaving a size of N_(—)0×N_(—)0, according to each partition type. Theprediction encoding in an intra mode and an inter mode may be performedon the partitions having the sizes of 2N_(—)0×2N_(—)0, N_(—)0×2N_(—)0,2N_(—)0×N_(—)0, and N_(—)0×N_(—)0. The prediction encoding in a skipmode is performed only on the partition having the size of2N_(—)0×2N_(—)0.

If an encoding error is the smallest in one of the partition types 912through 916 having the sizes of 2N_(—)0×2N_(—)0, 2N_(—)0×N_(—)0, andN_(—)0×2N_(—)0, the prediction unit 910 may not be split into a lowerdepth.

If the encoding error is the smallest in the partition type 918 havingthe size of N_(—)0×N_(—)0, a depth is changed from 0 to 1 to split thepartition type 918 in operation 920, and encoding is repeatedlyperformed on partition type coding units having a depth of 2 and a sizeof N_(—)0×N_(—)0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding of the partition typecoding unit 930 having a depth of 1 and a size of 2N_(—)1×2N_(—)1(=N_(—)0×N_(—)0) may include partitions of a partition type 942 having asize of 2N_(—)1×2N_(—)1, a partition type 944 having a size of2N_(—)1×N_(—)1, a partition type 946 having a size of N_(—)1×2N_(—)1,and a partition type 948 having a size of N_(—)1×N_(—)1.

If an encoding error is the smallest in the partition type 948 havingthe size of N_(—)1×N_(—)1, a depth is changed from 1 to 2 to split thepartition type 948 in operation 950, and encoding is repeatedlyperformed on coding units 960, which have a depth of 2 and a size ofN_(—)2×N_(—)2 to search for a minimum encoding error.

When a maximum depth is d, a split operation according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. In other words,when encoding is performed up to when the depth is d−1 after a codingunit corresponding to a depth of d−2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d−1 and a size of 2N_(d−1)×2N_(d−1) may include partitions of apartition type 992 having a size of 2N_(d−1)×2N_(d−1), a partition type994 having a size of 2N_(d−1)×N_(d−1), a partition type 996 having asize of N_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 having the size of N_(d−1)×N_(d−1) hasthe minimum encoding error, since a maximum depth is d, a coding unitCU_(d−1) having a depth of d−1 is no longer split to a lower depth, anda coded depth for the coding units constituting the current largestcoding unit 900 is determined to be d−1 and a partition type of thecurrent largest coding unit 900 may be determined to be N_(d−1)×N_(d−1).Also, since the maximum depth is d, split information for the smallestcoding unit 952 is not set.

A data unit 999 may be a ‘minimum unit’ for the current largest codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting the smallest coding unit 980by 4. By performing the encoding repeatedly, the video encodingapparatus 100 may select a depth having the least encoding error bycomparing encoding errors according to depths of the coding unit 900 todetermine a coded depth, and set a corresponding partition type and aprediction mode as an encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The entropy decoder 220 of the video decoding apparatus 200 may extractand use the information about the coded depth and the prediction unit ofthe coding unit 900 to decode the coding unit 912. The video decodingapparatus 200 may determine a depth, in which split information is 0, asa coded depth by using split information according to depths, and useinformation about an encoding mode of the corresponding depth fordecoding.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a largest coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and thetransformation units 1070 are transformation units of each of the codingunits 1010.

When a depth of a largest coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some coding units 1014, 1016, 1022, 1032,1048, 1050, 1052, and 1054 are obtained by splitting the coding units.In other words, partition types in the coding units 1014, 1022, 1050,and 1054 have a size of 2N×N, partition types in the coding units 1016,1048, and 1052 have a size of N×2N, and a partition type of the codingunit 1032 has a size of N×N. Prediction units and partitions of thecoding units 1010 are equal to or smaller than each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, 1052, and 1054 in the transformation units1070 are different from those in the prediction units 1060 in terms ofsizes and shapes. In other words, the video encoding apparatus 100 andthe video decoding apparatus 200 may perform intra prediction, motionestimation, motion compensation, transformation, and inversetransformation individually on a data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a largest coding unitto determine an optimum coding unit. Thus, coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit.

Table 1 shows the encoding information that may be set by the videoencoding apparatus 100 and the video decoding apparatus 200.

TABLE 1 Split Information 0 (Encoding on Coding Unit having Size of 2N ×2N and Current Depth of d) Size of Transformation Unit Split SplitPartition Type Information 0 Information 1 Symmetrical of of PredictionPartition Asymmetrical Transformation Transformation Split Mode TypePartition Type Unit Unit Information 1 Intra 2N × nU 2N × nU 2N × 2N N ×N Repeatedly Inter 2N × nD 2N × nD (Symmetrical Encode Skip (Only nL ×2N nL × 2N Partition Type) Coding Units 2N × 2N) nR × 2N nR × 2N N/2 ×N/2 Having Lower (Asymmetrical Depth of Partition Type) d + 1

The entropy encoder 120 of the video encoding apparatus 100 may outputthe encoding information about the coding units having a tree structure,and the entropy decoder 220 of the video decoding apparatus 200 mayextract the encoding information about the coding units having a treestructure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth. Thus, information about a partition type,a prediction mode, and a size of a transformation unit may be definedfor the coded depth. If the current coding unit is further splitaccording to the split information, encoding is independently performedon four split coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:n and n:1 (where n isan integer greater than 1), and the asymmetrical partition types havingthe sizes of nL×2N and nR×2N may be respectively obtained by splittingthe width of the prediction unit in 1:n and n:1.

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. In other words, if splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. If split information of the transformation unit is 1, thetransformation units may be obtained by splitting the current codingunit. Also, if a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and if the partition type of the currentcoding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit. Thus, a distribution of codeddepths in a largest coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoded information of the data units,and the searched adjacent coding units may be referred to for predictingthe current coding unit.

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit, and a transformation unit according to theencoding mode information of Table 1.

A largest coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, a transformation unit 1342 having a sizeof 2N×2N is set if split information (TU size flag) of a transformationunit is 0, and a transformation unit 1344 having a size of N×N is set ifa TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set if a TU size flag is 0, and a transformation unit 1354having a size of N/2×N/2 is set if a TU size flag is 1.

The TU size flag is a type of transformation index. A size of atransformation unit corresponding to a transformation index may bemodified according to a prediction unit type or a partition type of acoding unit.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, the transformation unit 1342 having asize of 2N×2N is set if a TU size flag of a transformation unit is 0,and the transformation unit 1344 having a size of N×N is set if a TUsize flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332 (2N×nU), 1334 (2N×nD), 1336 (nL×2N), or 1338 (nR×2N), thetransformation unit 1352 having a size of 2N×2N is set if a TU size flagis 0, and the transformation unit 1354 having a size of N/2×N/2 is setif a TU size flag is 1.

Referring to FIG. 9, the TU size flag described above is a flag having avalue of 0 or 1, but the TU size flag is not limited to 1 bit, and atransformation unit may be hierarchically split while the TU size flagincreases from 0. The transformation unit split information (TU sizeflag) may be used as an example of a transformation index.

In this case, when a TU size flag according to an embodiment is usedwith a maximum size and a minimum size of a transformation unit, thesize of the actually used transformation unit may be expressed. Thevideo encoding apparatus 100 may encode largest transformation unit sizeinformation, smallest transformation unit size information, and largesttransformation unit split information. The encoded largesttransformation unit size information, smallest transformation unit sizeinformation, and largest transformation unit split information may beinserted into a sequence parameter set (SPS). The video decodingapparatus 200 may use the largest transformation unit size information,the smallest transformation unit size information, and the largesttransformation unit split information for video decoding.

For example, (a) if a size of a current coding unit is 64×64 and alargest transformation unit is 32×32, (a-1) a size of a transformationunit is 32×32 if a TU size flag is 0; (a-2) a size of a transformationunit is 16×16 if a TU size flag is 1; and (a-3) a size of atransformation unit is 8×8 if a TU size flag is 2.

Alternatively, (b) if a size of a current coding unit is 32×32 and asmallest transformation unit is 32×32, (b−1) a size of a transformationunit is 32×32 if a TU size flag is 0, and since the size of atransformation unit cannot be smaller than 32×32, no more TU size flagsmay be set.

Alternatively, (c) if a size of a current encoding unit is 64×64 and amaximum TU size flag is 1, a TU size flag may be 0 or 1 and no other TUsize flags may be set.

Accordingly, when defining a maximum TU size flag as‘MaxTransformSizeIndex’, a minimum TU size flag as ‘MinTransformSize’,and a transformation unit in the case when a TU size flag is 0, that is,a root transformation unit RootTu as ‘RootTuSize’, a size of a smallesttransformation unit ‘CurrMinTuSize’, which is available in a currentcoding unit, may be defined by Equation (1) below.

CurrMinTuSize=max(MinTransformSize,RootTuSize/(2̂MaxTransformSizeIndex))  (1)

In comparison with the size of the smallest transformation unit‘CurrMinTuSize’ that is available in the current coding unit, the roottransformation unit size ‘RootTuSize’, which is a size of atransformation unit when if a TU size flag is 0, may indicate a largesttransformation unit which may be selected in regard to a system. Thatis, according to Equation (1), ‘RootTuSize/(2̂MaxTransformSizeIndex)’ isa size of a transformation unit that is obtained by splitting‘RootTuSize’, which is a size of a transformation unit whentransformation unit split information is 0, by the number of splittingtimes corresponding to the largest transformation unit splitinformation, and ‘MinTransformSize’ is a size of a smallesttransformation unit, and thus a smaller value of these may be‘CurrMinTuSize’ which is the size of the smallest transformation unitthat is available in the current coding unit.

The size of the root transformation unit ‘RootTuSize’ according to anexemplary embodiment may vary according to a prediction mode.

For example, if a current prediction mode is an inter mode, RootTuSizemay be determined according to Equation (2) below.

RootTuSize=min(MaxTransformSize,PUSize)  (2)

In Equation (2), ‘MaxTransformSize’ refers to a largest transformationunit size, and ‘PUSize’ refers to a current prediction unit size.

In other words, if a current prediction mode is an inter mode, the sizeof the root transformation unit size ‘RootTuSize’, which is atransformation unit if a TU size flag is 0, may be set to a smallervalue from among the largest transformation unit size and the currentprediction unit size.

If a prediction mode of a current partition unit is an intra mode,‘RootTuSize’ may be determined according to Equation (3) below.

RootTuSize=min(MaxTransformSize,PartitionSize)  (3)

In Equation (3), ‘PartitionSize’ refers to a size of the currentpartition unit.

In other words, if a current prediction mode is an intra mode, the roottransformation unit size ‘RootTuSize’ may be set to a smaller value fromamong the largest transformation unit size and the current partitionunit size.

However, it should be noted that the size of the root transformationunit size ‘RootTuSize’, which is the current largest transformation unitsize according to an exemplary embodiment and varies according to aprediction mode of a partition unit, is an example, and factors fordetermining the current largest transformation unit size are not limitedthereto.

An entropy encoding operation of a syntax element, which is performed bythe entropy encoder 120 of the video encoding apparatus 100 of FIG. 1,and an entropy decoding operation of a syntax element, which isperformed by the entropy decoder 220 of the video decoding apparatus 200of FIG. 2, will now be described.

As described above, the video encoding apparatus 100 and the videodecoding apparatus 200 perform encoding and decoding by splitting alargest coding unit into coding units that are equal to or smaller thanthe largest coding unit. A prediction unit and a transformation unitused in prediction and transformation may be determined based on costswhich are independent from other data units. Since an optimum codingunit may be determined by recursively encoding each coding unit having ahierarchical structure included in the largest coding unit, data unitshaving a tree structure may be configured. In other words, for eachlargest coding unit, a coding unit having a tree structure, and aprediction unit and a transformation unit each having a tree structuremay be configured. For decoding, hierarchical information, which isinformation indicating structure information of data units having ahierarchical structure, and non-hierarchical information for decoding,besides the hierarchical information, need to be transmitted.

The information related to a hierarchical structure is informationneeded for determining a coding unit having a tree structure, aprediction unit having a tree structure, and a transformation unithaving a tree structure, as described above with reference to FIGS. 10through 12. Further, the information related to the hierarchicalstructure includes size information of a largest coding unit, codeddepth, partition information of a prediction unit, a split flagindicating whether a coding unit is split, size information of atransformation unit, and a split transformation flag(split_transform_flag) indicating whether a transformation unit is splitinto smaller transformation units for a transformation operation.Examples of coding information other than hierarchical structureinformation include prediction mode information of intra/interprediction applied to each prediction unit, motion vector information,prediction direction information, color component information applied toeach data unit in the case when a plurality of color components areused, and transformation coefficient level information. Hereinafter,hierarchical information and extra-hierarchical information may bereferred to as a syntax element which is to be entropy encoded orentropy decoded.

In particular, according to exemplary embodiments, a method of selectinga context model when a syntax element related to a transformation unitfrom among syntax elements is provided. An operation of entropy encodingand decoding syntax elements related to a transformation unit will nowbe described in detail.

FIG. 14 is a block diagram of an entropy encoding apparatus 1400according to an exemplary embodiment. The entropy encoding apparatus1400 corresponds to the entropy encoder 120 of the video encodingapparatus 100 of FIG. 1.

Referring to FIG. 14, the entropy encoding apparatus 1400 includes abinarizer 1410, a context modeler 1420, and a binary arithmetic coder1430. Also, the binary arithmetic coder 1430 includes a regular codingengine 1432 and a bypass coding engine 1434.

When syntax elements input to the entropy encoding apparatus 1400 arenot binary values, the binarizer 1410 binarizes syntax elements so as tooutput a bin string consisting of binary values of 0 and 1. A bindenotes each bit of a stream consisting of 0 and 1, and is encoded bycontext adaptive binary arithmetic coding (CABAC). If a syntax elementis data having a same probability between 0 and 1, the syntax element isoutput to the bypass coding engine 1434, which does not use aprobability, to be encoded.

The binarizer 1410 may use various binarization methods according to thetype of a syntax element. Examples of the binarization methods mayinclude a unary method, a truncated unary method, a truncated rice codemethod, a Golomb code method, and a fixed length code method.

A transformation unit significant coefficient flag cbf indicatingwhether a non-zero transformation coefficient (hereinafter, referred toas a “significant coefficient”) exists in a transformation unit isbinarized by using the fixed code method. That is, if a non-zerotransformation coefficient exists in the transformation unit, thetransformation unit significant coefficient flag cbf is set to have avalue to 1. Otherwise, if a non-zero transformation coefficient does notexist in the transformation unit, the transformation unit significantcoefficient flag cbf is set to have a value of 0. If an image includes aplurality of color components, the transformation unit significantcoefficient flag cbf may be set with respect to a transformation unit ofeach color component. For example, if an image includes luma (Y) andchroma (Cb, Cr) components, a transformation unit significantcoefficient flag cbf_luma of a transformation unit of the lumacomponent, and a transformation unit significant coefficient flag cbf_cbor cbf_cr of the transformation unit of the chroma component may be set.

The context modeler 1420 provides a context model for encoding a bitstring corresponding to a syntax element, to the regular coding engine1432. In more detail, the context modeler 1420 outputs a probability ofa binary value for encoding each binary value of a bit string of acurrent syntax element, to the binary arithmetic coder 1430.

A context model is a probability model of a bin, and includesinformation about which of 0 and 1 corresponds to a most probable symbol(MPS) and a least probable symbol (LPS), and probability information ofat least one of the MPS and the LPS.

The context modeler 1420 may select a context model for entropy encodingthe transformation unit significant coefficient flag cbf, based on atransformation depth of the transformation unit. If the size of thetransformation unit is equal to the size of a coding unit, that is, ifthe transformation depth of the transformation unit is 0, the contextmodeler 1420 may determine a preset first context model as a contextmodel for entropy encoding the transformation unit significantcoefficient flag cbf. Otherwise, if the size of the transformation unitis less than the size of the coding unit, that is, if the transformationdepth of the transformation unit is not 0, the context modeler 1420 maydetermine a preset second context model as a context model for entropyencoding the transformation unit significant coefficient flag cbf. Here,the first and second context models are based on different probabilitydistribution models. That is, the first and second context models aredifferent context models.

As described above, when the transformation unit significant coefficientflag cbf is entropy encoded, the context modeler 1420 uses differentcontext models in a case when the size of the transformation unit isequal to the size of the coding unit, and a case when the size of thetransformation unit is not equal to the size of the coding unit. If anindex indicating one of a plurality of preset context models for entropyencoding the transformation unit significant coefficient flag cbf isreferred to as a context index ctxIdx, the context index ctxIdx may havea value obtained by summing a context increasement parameter ctxInc fordetermining a context model, and a preset context index offsetctxIdxOffset. That is, ctxIdx=ctxInc+ctxIdxOffset. The context modeler1420 may distinguish a case when the transformation depth of thetransformation unit is 0 from a case when the transformation depth ofthe transformation unit is not 0, may change the context increasementparameter ctxInc for determining a context model, based on thetransformation depth of the transformation unit, and thus may change thecontext index ctxIdx for determining a context model for entropyencoding the transformation unit significant coefficient flag cbf.

In more detail, if the transformation depth is referred to astrafodepth, the context modeler 1420 may determine the contextincreasement parameter ctxInc based on the following algorithm.

ctxInc=(trafodepth==0)?1:0

This algorithm may be implemented by the following pseudo code.

{ If (trafodepth==0) ctxInc=1; else ctxInc=0; }

The transformation unit significant coefficient flag cbf may beseparately set according to luma and chroma components. As describedabove, a context model for entropy encoding the transformation unitsignificant coefficient flag cbf_luma of the transformation unit of theluma component may be determined by using the context increasementparameter ctxInc that changes according to whether the transformationdepth of the transformation unit is 0. A context model for entropyencoding the transformation unit significant coefficient flag cbf_cb orcbf_cr of the transformation unit of the chroma component may bedetermined by using a value of the transformation depth trafodepth asthe context increasement parameter ctxInc.

The regular coding engine 1432 performs binary arithmetic encoding on abitstream corresponding to a syntax element, based on the informationabout the MPS and the LPS and the probability information of at leastone of the MPS and the LPS, which are included in the context modelprovided from the context modeler 1420.

FIG. 15 is a flowchart of an operation of entropy encoding and decodinga syntax element related to a transformation unit, according to anexemplary embodiment.

Referring to FIG. 15, in operation 1510, a transformation unitsignificant coefficient flag cbf indicating whether a non-zerotransformation coefficient exists from among transformation coefficientsincluded in a current transformation unit is initially entropy encodedand decoded. As described above, a context model for entropy encodingthe transformation unit significant coefficient flag cbf may bedetermined based on a transformation depth of the transformation unit,and binary arithmetic encoding on the transformation unit significantcoefficient flag cbf may be performed based on the determined contextmodel.

If the transformation unit significant coefficient flag cbf is 0, sinceonly transformation coefficients of 0 exist in the currenttransformation unit, only a value 0 is entropy encoded or decoded as thetransformation unit significant coefficient flag cbf, and transformationcoefficient level information is not entropy encoded or decoded.

In operation 1520, if a significant coefficient exists in the currenttransformation unit, a significance map SigMap indicating a location ofa significant coefficient is entropy encoded or decoded.

A significance map SigMap may be formed of a significant bit andpredetermined information indicating a location of a last significancecoefficient. A significant bit indicates whether a transformationcoefficient according to each scan index is a significant coefficient or0, and may be expressed by significant_coeff_flag[i]. As will bedescribed below, a significance map is set in units of subsets having apredetermined size which is obtained by splitting the transformationunit. Accordingly, significant_coeff_flag[i] indicates whether atransformation coefficient of an i-th scan index from amongtransformation coefficients included in a subset included in thetransformation unit is 0.

According to the related art H.264, a flag (End-Of-Block) indicatingwhether each significant coefficient is the last significant coefficientis separately entropy encoded or decoded. However, according to anexemplary embodiment, location information of the last significantcoefficient itself is entropy encoded or decoded. For example, if alocation of the last significant coefficient is (x, y), where x and yare integers, last_significant_coeff_x and last_significant_coeff_ywhich are syntax elements indicating coordinate values of (x, y) may beentropy encoded or decoded.

In operation 1530, transformation coefficient level informationindicating a size of a transformation coefficient is entropy encoded ordecoded. According to the related art H.264/AVC, transformationcoefficient level information is expressed by coeff_abs_level_minus 1which is a syntax element. According to exemplary embodiments, astransformation coefficient level information,coeff_abs_level_greater1_flag which is a syntax element regardingwhether an absolute value of a transformation coefficient is greaterthan 1, coeff_abs_level_greater2_flag which is a syntax elementregarding whether an absolute value of a transformation coefficient isgreater than 2, and coeff_abs_level_remaining which indicates sizeinformation of the remaining transformation coefficient are encoded.

The syntax element coeff_abs_level_remaining indicating the sizeinformation of the remaining transformation coefficient has a differencein a range between a size of a transformation coefficient (absCoeff) anda base level value baseLevel that is determined by usingcoeff_abs_level_greater1_flag and coeff_abs_level_greater2_flag. Thebase level value baseLevel is determined according to the equation:baseLevel=1+coeff_abs_level_greather1_flag+coeff_abs_level_greather2_flag,and coeff_abs_level_remaining is determined according to the equation:coeff_abs_level_remaining=absCoeff-baseLevel. Whilecoeff_abs_level_greater1_flag and coeff_abs_level_greater2_flag have avalue of 0 or 1, the base level value baseLevel may have a value from 1to 3. Accordingly, coeff_abs_level_remaining may be varied from(absCoeff-1) to (absCoeff-3). As described above, (absCoeff-baseLevel),which is a difference between the size of an original transformationcoefficient absCoeff and the base level value baseLevel, is transmittedas size information of a transformation coefficient in order to reduce asize of transmitted data.

An operation of determining a context model for entropy encoding atransformation unit significant coefficient flag, according to anexemplary embodiment, will now be described.

FIG. 16 is a diagram illustrating a coding unit and transformation units1611 through 1617 included in the coding unit, according to an exemplaryembodiment. In FIG. 16, a data unit indicated by a dashed line denotesthe coding unit (CU), and data units indicated by solid lines denote thetransformation units (TU) 1611 through 1617.

As described above, the video encoding apparatus 100 and the videodecoding apparatus 200 performs encoding and decoding by splitting alargest coding unit into coding units having a size equal to or lessthan the size of the largest coding unit. A prediction unit and atransformation unit used in a prediction operation and a transformationoperation may be determined based on costs which are independent fromother data units. If the size of a coding unit is greater than the sizeof a largest transformation unit usable by the video encoding apparatus100 and the video decoding apparatus 200, the coding unit may be splitinto transformation units having a size equal to or less than the sizeof the largest transformation unit, and a transformation operation maybe performed based on the split transformation units. For example, ifthe size of a coding unit is 64×64 and the size of a usable largesttransformation unit is 32×32, in order to transform (or inverselytransform) the coding unit, the coding unit is split into transformationunits having a size equal to or less than 32×32.

A transformation depth (trafodepth) indicating the number of times thecoding unit is split in horizontal and vertical directions intotransformation units may be determined. For example, if the size of acurrent coding unit is 2N×2N and the size of the transformation unit ofis 2N×2N, the transformation depth may be determined as 0. If the sizeof the transformation unit is N×N, the transformation depth may bedetermined as 1. Otherwise, if the size of the transformation unit isN/2×N/2, the transformation depth may be determined as 2.

Referring to FIG. 16, the transformation units 1611, 1616, and 1617 arelevel-1 transformation units obtained by splitting a root coding unitonce, and have a transformation depth of 1. The transformation units1612, 1614, 1614, and 1615 are level-2 transformation units obtained bysplitting a level-1 transformation unit into four pieces, and have atransformation depth of 2.

FIG. 17 is a diagram illustrating a context increasement parameterctxInc used to determine a context model of a transformation unitsignificant coefficient flag cbf of each of the transformation units1611 through 1617 of FIG. 16, based on a transformation depth. In thetree structure of FIG. 17, leaf nodes 1711 through 1717 respectivelycorrespond to the transformation units 1611 through 1617 of FIG. 16, andvalues of 0 and 1 marked on the leaf nodes 1711 through 1717 indicatethe transformation unit significant coefficient flags cbf of thetransformation units 1611 through 1617. Also, in FIG. 17, leaf nodeshaving the same transformation depth are illustrated in the order oftransformation units located at top left, top right, bottom left, andbottom right sides. For example, the leaf nodes 1712, 1713, 1714, and1715 of FIG. 17 respectively correspond to the transformation units1612, 1613, 1614, and 1615 of FIG. 16. Also, referring to FIGS. 16 and17, it is assumed that only the transformation unit significantcoefficient flags cbf of the transformation units 1612 and 1614 are 1,and that the transformation unit significant coefficient flags cbf ofthe other transformation units are 0.

Referring to FIG. 17, since all of the transformation units 1611 through1617 of FIG. 16 are obtained by splitting a root coding unit and thushave non-zero transformation depths, the context increasement parameterctxInc used to determine a context model of the transformation unitsignificant coefficient flag cbf of each of the transformation units1611 through 1617 is set to have a value of 0.

FIG. 18 is a diagram illustrating a coding unit 1811 and atransformation unit 1812 included in the coding unit 1811, according toanother exemplary embodiment. In FIG. 18, a data unit indicated by adashed line denotes the coding unit 1811, and a data unit indicated by asolid line denotes the transformation unit 1812.

Referring to FIG. 18, if the size of the coding unit 1811 is equal tothe size of the transformation unit 1812 used to transform the codingunit 1811, a transformation depth (trafodepth) of the transformationunit 1812 has a value of 0. If the transformation unit 1812 has atransformation depth of 0, a context increasement parameter ctxInc usedto determine a context model of a transformation unit significantcoefficient flag cbf of the transformation unit 1812 is set to have avalue of 1.

The context modeler 1420 of FIG. 14 may compare the size of a codingunit to the size of a transformation unit based on a transformationdepth of the transformation unit, may distinguish a case when thetransformation depth of the transformation unit is 0 from a case whenthe transformation depth of the transformation unit is not 0, and thusmay change the context increasement parameter ctxInc used to determine acontext model for entropy encoding the transformation unit significantcoefficient flag cbf. By changing the context increasement parameterctxInc used to determine a context model, the context model for entropyencoding the transformation unit significant coefficient flag cbf may bechanged in a case when the transformation depth of the transformationunit is 0 and a case when the transformation depth of the transformationunit is not 0.

FIG. 19 is a diagram illustrating split transformation flagssplit_transform_flag used to determine the structure of transformationunits included in the coding unit of FIG. 16, according to an exemplaryembodiment.

The video encoding apparatus 100 may signal information about thestructure of transformation units used to transform each coding unit, tothe video decoding apparatus 200. The information about the structure oftransformation units may be signaled by using the split transformationflag split_transform_flag indicating whether each coding unit is splitin horizontal and vertical directions into four transformation units.

Referring to FIGS. 16 and 19, since a root coding unit is split intofour pieces, a split transformation flag split_transform_flag 1910 ofthe root coding unit is set as 1. If the size of the root coding unit isgreater than the size of a usable largest transformation unit, the splittransformation flag split_transform_flag 1910 of the root coding unitmay always be set as 1 and may not be signaled. Therefore, if the sizeof a coding unit is greater than the size of a usable largesttransformation unit, the coding unit does not need to be split intodeeper coding units having a size equal to or less than the size of atleast a largest transformation unit.

With respect to each of the four transformation units split from theroot coding unit had having a transformation depth of 1, a splittransformation flag indicating whether to split each of the fourtransformation units into four transformation units having atransformation depth of 2 is set. In FIG. 19, split transformation flagsof the transformation units having the same transformation depth areillustrated in the order of the transformation units located at topleft, top right, bottom left, and bottom right sides. A referencenumeral 1911 denotes a split transformation flag of the transformationunit 1611 of FIG. 16. Since the transformation unit 1611 is not splitinto transformation units having a lower depth, the split transformationflag 1911 of the transformation unit 1611 has a value of 0. Likewise,since the transformation units 1616 and 1617 of FIG. 16 are not splitinto transformation units having a lower depth, split transformationflags 1913 and 1914 of the transformation units 1616 and 1617 have avalue of 0. Since the top right transformation unit having atransformation depth of 1 in FIG. 16 is split into the transformationunits 1612, 1613, 1614, and 1615 having a transformation depth of 2, asplit transformation flag 1912 of the top right transformation unit hasa transformation depth of 1. Since the transformation units 1612, 1613,1614, and 1615 having a transformation depth of 2 are not split intotransformation units having a lower depth, split transformation flags1915, 1916, 1917, and 1918 of the transformation units 1612, 1613, 1614,and 1615 having a transformation depth of 2 have a value of 0.

As described above, a context model for entropy encoding atransformation unit significant coefficient flag cbf may be determinedbased on a transformation depth of a transformation unit, and binaryarithmetic encoding may be performed on the transformation unitsignificant coefficient flag cbf based on the selected context model. Ifthe transformation unit significant coefficient flag cbf is 0, sinceonly 0 transformation coefficients exist in a current transformationunit, only a value of 0 is entropy encoded or decoded as thetransformation unit significant coefficient flag cbf, and transformationcoefficient level information is not entropy encoded or decoded.

An operation of entropy encoding a syntax element related totransformation coefficients included in a transformation unit of which atransformation unit significant coefficient flag cbf has a value of 1,that is, a transformation unit having a non zero transformationcoefficient, will now be described.

FIG. 20 illustrates a transformation unit 2000 that is entropy encodedaccording to an exemplary embodiment. While the transformation unit 2000having a 16×16 size is illustrated in FIG. 20, the size of thetransformation unit 2000 is not limited to the illustrated size of 16×16but may also be of various sizes ranging from 4×4 to 32×32.

Referring to FIG. 20, for entropy encoding and decoding of thetransformation coefficient included in the transformation unit 2000, thetransformation unit 2000 may be divided into smaller transformationunits. An operation of entropy encoding a syntax element related to a4×4 transformation unit 2010 included in the transformation unit 2000will now be described. This entropy encoding operation may also beapplied to a transformation unit of different sizes.

Transformation coefficients included in the 4×4 transformation unit 2010each have a transformation coefficient (absCoeff) as illustrated in FIG.20. The transformation coefficients included in the 4×4 transformationunit 2010 may be serialized according to a predetermined scanning orderas illustrated in FIG. 20 and sequentially processed. However, thescanning order is not limited as illustrated but may also be modified.

Examples of syntax elements related to transformation coefficientsincluded in the 4×4 transformation unit 2010 are significant_coeff_flagwhich is a syntax element indicating whether each transformationcoefficient included in a transformation unit is a significantcoefficient having a value that is not 0, coeff_abs_level_greater1_flagwhich is a syntax element indicating whether an absolute value of thetransformation coefficient is greater than 1,coeff_abs_level_greater2_flag which is a syntax element indicatingwhether the absolute value s greater than 2, andcoeff_abs_level_remaining which is a syntax element indicating sizeinformation of the remaining transformation coefficients.

FIG. 21 illustrates a significance map SigMap 2100 corresponding to thetransformation unit 2010 of FIG. 20.

Referring to FIGS. 20 and 21, the significance map SigMap 2100 having avalue of 1 for each of the significant coefficients that have a valuethat is not 0, from among the transformation coefficients included inthe 4×4 transformation unit 2010 of FIG. 20, is set. The significancemap SigMap 2100 is entropy encoded or decoded by using a previously setcontext model.

FIG. 22 illustrates coeff_abs_level_greater1_flag 2200 corresponding tothe 4×4 transformation unit 2010 of FIG. 20.

Referring to FIGS. 20 through 22, the coeff_abs_level_greater1_flag 2200which is a flag indicating whether a corresponding significancetransformation coefficient has a value greater than 1, regardingsignificant coefficients for which the significance map SigMap 2100 hasa value of 1, is set. When the coeff_abs_level_greater1_flag 2200 is 1,it indicates that a corresponding transformation coefficient is atransformation coefficient having a value greater than 1, and when thecoeff_abs_level_greater1_flag 2200 is 0, it indicates that acorresponding transformation coefficient is a transformation coefficienthaving a value of 1. In FIG. 22, when coeff_abs_level_greater1_flag 2210is at a location of a transformation coefficient having a value of 1,the coeff_abs_level_greater1_flag 2210 has a value of 0.

FIG. 23 illustrates coeff_abs_level_greater2_flag 2300 corresponding tothe 4×4 transformation unit 2010 of FIG. 20.

Referring to FIGS. 20 through 23, coeff_abs_level_greater2_flag 2300indicating whether a corresponding transformation coefficient has avalue greater than 2, regarding transformation coefficients for whichthe coeff_abs_level_greater1_flag 2200 is set to 1, is set. When thecoeff_abs_level_greater2_flag 2300 is 1, it indicates that acorresponding transformation coefficient is a transformation coefficienthaving a value greater than 2, and when thecoeff_abs_level_greater2_flag 2300 is 0, it indicates that acorresponding transformation coefficient is a transformation coefficienthaving a value of 2. In FIG. 23, when coeff_abs_level_greater2_flag 2310is at a location of a transformation coefficient having a value of 2,the coeff_abs_level_greater2_flag 2310 has a value of 0.

FIG. 24 illustrates coeff_abs_level_remaining 2400 corresponding to the4×4 transformation unit 2010 of FIG. 20.

Referring to FIGS. 20 through 24, the coeff_abs_level_remaining 2400which is a syntax element indicating size information of the remainingtransformation coefficients may be obtained by calculating(absCoeff-baseLevel) of each transformation coefficient.

The coeff_abs_level_remaining 2400 which is the syntax elementindicating size information of the remaining transformation coefficientshas a difference in a range between the size of the transformationcoefficient (absCoeff) and a base level value baseLevel determined byusing coeff_abs_level_greater1_flag and coeff_abs_level_greater2_flag.The base level value baseLevel is determined according to the equationbelow:

baseLevel=1+coeff_abs_level_greather1_flag+coeff_abs_level_greather2_flag,and coeff_abs_level_remaining is determined according to the equation:coeff_abs_level_remaining=absCoeff-baseLevel.

The coeff_abs_level_remaining 2400 may be read and entropy encodedaccording to the illustrated scanning order.

FIG. 25 is a flowchart of an entropy encoding method of a video,according to an exemplary embodiment.

Referring to FIGS. 14 and 25, in operation 2510, the context modeler1420 obtains data of a coding unit transformed based on a transformationunit. In operation 2520, the context modeler 1420 determines a contextmodel for arithmetically encoding a transformation unit significantcoefficient flag indicating whether a non-zero transformationcoefficient exists in the transformation unit, based on a transformationdepth of the transformation unit.

The context modeler 1420 may determine different context models in acase when the size of the transformation unit equals to the size of thecoding unit, that is, when the transformation depth of thetransformation unit is 0, and a case when the size of the transformationunit is less than the size of the coding unit, that is, when thetransformation depth of the transformation unit is not 0. In moredetail, the context modeler 1420 may change a context increasementparameter ctxInc for determining a context model, based on thetransformation depth of the transformation unit, may distinguish a casewhen the transformation depth of the transformation unit is 0 from acase when the transformation depth of the transformation unit is not 0,and thus may change a context index ctxIdx for determining a contextmodel for entropy encoding a transformation unit significant coefficientflag.

The transformation unit significant coefficient flag may be separatelyset according to luma and chroma components. A context model for entropyencoding a transformation unit significant coefficient flag cbf_luma ofthe transformation unit of the luma component may be determined by usingthe context increasement parameter ctxInc changed according to whetherthe transformation depth of the transformation unit is 0. A contextmodel for entropy encoding a transformation unit significant coefficientflag cbf_cb or cbf_cr of the transformation unit of the chroma componentmay be determined by using a value of the transformation depth(trafodepth) as the context increasement parameter ctxInc.

In operation 2530, the regular coding engine 1432 arithmetically encodesthe transformation unit significant coefficient flag based on thedetermined context model.

FIG. 26 is a block diagram of an entropy decoding apparatus 2600according to an exemplary embodiment. The entropy decoding apparatus2600 corresponds to the entropy decoder 220 of the video decodingapparatus 200 of FIG. 2. The entropy decoding apparatus 2600 performs aninverse operation of the entropy encoding operation performed by theentropy encoding apparatus 1400 described above.

Referring to FIG. 26, the entropy decoding apparatus 2600 includes acontext modeler 2610, a regular decoding engine 2620, a bypass decodingengine 2630, and a de-binarizer 2640.

A syntax element encoded by using bypass coding is output to the bypassdecoder 2630 to be arithmetically decoded, and a syntax element encodedby using regular coding is arithmetically decoded by the regular decoder2620. The regular decoder 2620 arithmetically decodes a binary value ofa current syntax element based on a context model provided by using thecontext modeler 2610 to thereby output a bit string.

Like the above-described context modeler 1420 of FIG. 14, the contextmodeler 2610 may select a context model for entropy decoding atransformation unit significant coefficient flag cbf, based on atransformation depth of a transformation unit. That is, the contextmodeler 2610 may determine different context models in a case when thesize of the transformation unit equals to the size of a coding unit,that is, when the transformation depth of the transformation unit is 0,and a case when the size of the transformation unit is less than thesize of the coding unit, that is, when the transformation depth of thetransformation unit is not 0. In more detail, the context modeler 2610may change a context increasement parameter ctxInc for determining acontext model, based on the transformation depth of the transformationunit, may distinguish a case when the transformation depth of thetransformation unit is 0 from a case when the transformation depth ofthe transformation unit is not 0, and thus may change a context indexctxIdx for determining a context model for entropy decoding thetransformation unit significant coefficient flag cbf.

If the structure of transformation units included in a coding unit isdetermined based on a split transformation flag split_transform_flagindicating whether a coding unit obtained from a bitstream is split intothe transformation units, the transformation depth of the transformationunit may be determined based on the number of times the coding unit issplit to reach the transformation unit.

The transformation unit significant coefficient flag cbf may beseparately set according to luma and chroma components. A context modelfor entropy decoding a transformation unit significant coefficient flagcbf_luma of the transformation unit of the luma component may bedetermined by using the context increasement parameter ctxInc thatchanges according to whether the transformation depth of thetransformation unit is 0. A context model for entropy decoding atransformation unit significant coefficient flag cbf_cb or cbf_cr of thetransformation unit of the chroma component may be determined by using avalue of the transformation depth (trafodepth) as the contextincreasement parameter ctxInc.

The de-binarizer 2640 reconstructs bit strings that are arithmeticallydecoded by the regular decoding engine 2620 or the bypass decodingengine 2630, to syntax elements again.

The entropy decoding apparatus 2600 arithmetically decodes syntaxelements related to transformation units, such ascoeff_abs_level_remaining, SigMap, coeff_abs_level_greater1_flag, andcoeff_abs_level_greater2_flag in addition to the transformation unitsignificant coefficient flag cbf, and outputs the same. When the syntaxelements related to a transformation unit are reconstructed, dataincluded in the transformation units may be decoded by using inversequantization, inverse transformation, and predictive decoding, based onthe reconstructed syntax elements.

FIG. 27 is a flowchart of an entropy decoding method of a video,according to an exemplary embodiment.

Referring to FIG. 27, in operation 2710, a transformation unit includedin a coding unit and used to inversely transform the coding unit isdetermined. As described above, the structure of transformation unitsincluded in a coding unit may be determined based on a splittransformation flag split_transform_flag indicating whether the codingunit obtained from a bitstream is split into transformation units. Also,a transformation depth of the transformation unit may be determinedbased on the number of times that the coding unit is split to reach thetransformation unit.

In operation 2720, the context modeler 2610 obtains a transformationunit significant coefficient flag indicating whether a non-zerotransformation coefficient exists in the transformation unit, from thebitstream.

In operation 2730, the context modeler 2610 determines a context modelfor arithmetically decoding the transformation unit significantcoefficient flag, based on the transformation depth of thetransformation unit. As described above, the context modeler 2610 maydetermine different context models in a case when the size of thetransformation unit equals to the size of the coding unit, that is, whenthe transformation depth of the transformation unit is 0, and a casewhen the size of the transformation unit is less than the size of thecoding unit, that is, when the transformation depth of thetransformation unit is not 0. In more detail, the context modeler 2610may change a context increasement parameter ctxInc for determining acontext model, based on the transformation depth of the transformationunit, may distinguish a case when the transformation depth of thetransformation unit is 0 from a case when the transformation depth ofthe transformation unit is not 0, and thus may change a context indexctxIdx for determining a context model for entropy decoding atransformation unit significant coefficient flag.

In operation 2740, the regular decoding engine 2620 arithmeticallydecodes the transformation unit significant coefficient flag based onthe context model provided from the context modeler 2610.

The above exemplary embodiments can also be embodied as computerreadable code on a computer readable recording medium. The computerreadable recording medium is any data storage device that can store datawhich can be thereafter read by a computer system. Examples of thecomputer readable recording medium include read-only memory (ROM),random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, andoptical data storage devices. The computer readable recording medium canalso be distributed over network coupled computer systems so that thecomputer readable code is stored and executed in a distributed fashion.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby one of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. An apparatus of decoding video, the apparatus comprising: a parserwhich obtains, from a bitstream, a split transformation flag used todetermine a transformation unit for inverse transformation process, andobtains at least one transformation unit from a coding unit based on thesplit transformation flag; a context modeler which obtains a contextmodel for arithmetic decoding a transformation unit significantcoefficient flag indicating whether a non-zero transformationcoefficient exists in a transformation unit included in a coding unit,based on a transformation depth of the transformation unit; and anarithmetic decoder which arithmetic decodes the transformation unitsignificant coefficient flag based on the determined context model.